Wire saw cutting method synchronizing workpiece feed speed with wire speed

ABSTRACT

A wire saw slicing method capable of slicing a workpiece into wafers having a flat shape free from undulations (waviness) includes feeding the workpiece or a wire along a linear feed path to force them together, and reciprocating the wire in the axial direction. A first cyclic pattern of the feed speed of the workpiece or the wire and a second cyclic pattern of the reciprocating speed of the wire are set in such a relation that the first and second cyclic patterns are synchronized at least during a time period of one of the modes of the respective cyclic patterns.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to a wire saw slicing apparatus and aslicing method for cutting off or slicing a workpiece, such as a siliconsemiconductor single crystal ingot, into wafer-like slices with nosurface irregularities and kerf loss involved and with good yieldresulting therefrom.

2. Description of the Related Art:

Wire saws are used for cutting off or slicing a workpiece, such as asemiconductor single crystal ingot, into wafers by reciprocating aplurality of parallel regularly spaced lines of wire in the axialdirection while feeding the workpiece relatively to the wire along alinear path to force the workpiece against the wire, with anabrasive-containing working fluid continuously supplied to a contactarea between the workpiece and the wire lines. The wire saws areadvantageous in that a multiplicity of wafers of the same thickness canbe produced at one time. A slicing time required for each slicing run oroperation is about 6 hours for a silicon semiconductor ingot having adiameter of 5 inches. In an alternative arrangement, the wire, like theworkpiece in the foregoing arrangement, is fed relatively to theworkpiece along a linear path to force the wire against the workpiece.In a description given below, only the first-mentioned arrangementincorporating a linear movement of the workpiece (in a downwarddirection, in general) will be described. It is obvious, however, thatthe second-mentioned arrangement incorporating a linear movement of thewire can be applied to in the same manner as the first-mentionedarrangement.

The sliced wafers ideally have the shape of a completely flat disk. Inreality, however, their shape deviates from the ideal shape and are mostlikely to have a deformation called "sori" (warp or bow) observed,respectively, as a concave or a convex and an S-shaped deformationhaving a relatively large periodicity or radius of curvature, or adeformation called "undulations (waviness)" appearing an surfaceirregularities having a periodicity of several millimeters to severaltens millimeters. The deformation of "sori" (warp or bow) and thedeformation of undulations (waviness) cannot be removed by a subsequentwafer-finishing process such as lapping process because the wafersundergo elastic deformation under loads exerted thereon during thelapping process. In view of progressive microminiaturization of theelectronic circuit, the deformation of "sori" (warp or bow) andundulations (waviness) of the as sliced wafers affects the yield ofsemiconductor devices formed on the wafers.

The deformation of "sori" (warp or bow) is generated due to variationsof pitch of a wire wound around main rollers of the wire saw slicingmachine, which are caused due to thermal expansion of the main rollersunder frictional heat generated during the slicing process. To deal withthis problem, various means for cooling the main rollers during theslicing process have been proposed.

The proposed method is able to preclude the generation of thedeformation of "sori" (warp or bow) by cooling the main rollers forkeeping the constant wire pitch. However, the deformation of undulations(waviness) on the as-sliced wafer cannot be avoided by the proposedmethod because the deformation of undulations (waviness) is consideredto be generated under the effect of variations of feed speed of theworkpiece relative to the wire taken in combination with temporary stopsat opposite ends of reciprocating movement of the wire where the wirechanges its direction of movement.

SUMMARY OF THE INVENTION

With the foregoing difficulties in view, it is an object of the presentinvention to provide a wire saw slicing apparatus and a wire saw slicingmethod which are capable of slicing a workpiece, such an a siliconsemiconductor single crystal ingot, into waters having a uniform shapefree from undulations (waviness).

In brief, according to a wire saw slicing apparatus and a wire sawslicing method of the present invention, the linear feed speed of theworkpiece relative to the wire is regulated in synchronism with thereciprocating movement of the wire recurring in a cycle pattern ofstopping, accelerating, continuing at a constant speed, decelerating,and stopping, or of stopping, accelerating, decelerating and stopping,so that the workpiece in sliced are the workpiece is fed linearlythrough a time period during which the wire is reciprocating at asufficient speed for slicing.

In one aspect the invention provides a wire saw slicing apparatus of thetype in which a plurality of parallel, regularly spaced lines of a wireand a workpiece are relatively fed toward each other alone a linear feedpath to force them together while reciprocating the wire in the axialdirection to slice the workpiece into wafers, with a slurry supplied toa contact area between the wire and the workpiece, the wire saw slicingapparatus comprising: (a) means for feeding one of the workpiece and thewire relative to the other along the linear feed path to force themtogether (b) means for reciprocating the wire in the axial direction (c)means for setting a cycle pattern of the feed speed of the workpiece orthe wire which recurs in a sequence of stopping, accelerating,continuing at a constant speed, decelerating, and stopping, and a cyclepattern of the reciprocating speed of the wire which recurs in asequence of stopping, accelerating, continuing at a constant speed,decelerating, and stopping, the two cycle patterns being synchronizedwith each other at least through a time period during which the feedspeed and the reciprocating speed continue at the respective constantspeeds; and (d) means for controlling the operation of the feeding meansand the reciprocating means according to the first and second cyclepatterns met by the setting means.

In a second preferred form of the present invention, there is provided awire saw slicing apparatus of the type in which a plurality of parallel,regularly spaced lines of a wire and a workpiece are relatively fedtoward each other along a linear feed path to force them together whilereciprocating the wire in the axial direction to slice the workpieceinto wafers, with a slurry supplied to a contact area between the wireand the workpiece, the wire saw slicing apparatus comprising: (a) meansfor feeding one of the workpiece and the wire relative to the otheralong the linear feed path to force them together (b) means forreciprocating the wire in the axial direction; (c) means for setting acycle pattern of the feed speed of the workpiece or the wire whichrecurs in a sequence of stopping, accelerating, continuing at a constantspeed, decelerating, and stopping, and a cycle pattern of thereciprocating speed of the wire which recurs in a sequence of stopping,accelerating, continuing at a constant speed, decelerating, andstopping, the two cycle patterns being synchronized with each other atleast at the time of the stopping of the respective speeds; and (d)means for controlling the operation of the feeding means and thereciprocating means according to the first and second cycle patterns setby the setting means.

In the first and second preferred forms of the present invention, thetwo cycle patterns are preferably synchronized with each other at leastat the time of the stopping of the respective speeds and through a timeperiod during which the feed speed and the reciprocating speed continueat the respective constant speeds.

In a third preferred form of the present invention, there is provided awire saw slicing apparatus of the type in which a plurality of parallel,regularly spaced lines of a wire and a workpiece are relatively fedtoward each other along a linear feed path to force them together whilereciprocating the wire in the axial direction to slice the workpieceinto wafers, with a slurry supplied to a contact area between the wireand the workpiece, the wire saw slicing apparatus comprising: (a) meansfor feeding one of the workpiece and the wire relative to the otheralong the linear feed path to force them together; (b) means forreciprocating the wire in the axial direction; (a) means for setting acycle pattern of the feed speed of the workpiece or the wire whichrecurs in a sequence of stopping, accelerating, decelerating, andstopping, and a cycle pattern of the reciprocating speed of the wirewhich recurs in a sequence of stopping, accelerating, decelerating, andstopping, the two cycle patterns being synchronized with each other atleast at the time of the stopping of the respective speeds; and (d)means for controlling the operation of the feeding means and thereciprocating means according to the first and second cycle patterns setby the setting means.

In the third preferred form, at least one of the two cycle patterns mayfurther have a mode of continuing at a constant speed provided betweenthe accelerating and the decelerating.

In any of the preferred forms previously described, the two cyclepatterns set by the setting means are preferably composed of arepetition of a unit pattern. Furthermore, it is preferable that each ofthe two cycle patterns set by the setting means are basically composedof a repetition of a unit pattern, and wherein the feed speed of theworkpiece or the wire in the continuing at a constant speed mode or theaccelerating mode varies inversely with a contact length between theworkpiece and the wire.

In a fourth preferred form of the present invention, there in provided awire saw slicing apparatus of the type in which a plurality of parallel,regularly spaced lines of a wire and a workpiece are relatively fedtoward each other along a linear feed path to force them together whilereciprocating the wire in the axial direction to slice the workpieceinto wafers, with a slurry supplied to a contact area between the wireand the workpiece, the wire saw slicing apparatus comprising: (a) meansfor feeding one of the workpiece and the wire relative to the otheralong the linear food path to force them together; (b) means forreciprocating the wire in the axial direction; (c) means for setting acycle pattern of the feed speed of the workpiece or the wire whichrecurs in a sequence of continuing at a constant speed, accelerating,continuing at the constant speed, decelerating, and continuing at theconstant speed, and a cycle pattern of the reciprocating speed of thewire which recurs in a sequence of stopping, accelerating, continuing ata constant speed, decelerating, and stopping, the two cycle patternsbeing correlated with each other such that at least a time period duringwhich the feed speed is in the continuing at the constant speed mode onthe high speed side and a time period during which the reciprocatingspeed is in the continuing at the constant speed mode are synchronizedwith each other; and (d) means for controlling the operation of thefeeding means and the reciprocating means according to the first andsecond cycle patterns set by the setting means.

In a fifth preferred form of the present invention, there is provided awire saw slicing apparatus of the type in which a plurality of parallel,regularly spaced lines of a wire and a workpiece are relatively fedtoward each other along a linear feed path to force them together whilereciprocating the wire in the axial direction to slice the workpieceinto wafers, with a slurry supplied to a contact area between the wireand the workpiece, the wire saw slicing apparatus comprising: (a) meansfor feeding one of the workpiece and the wire relative to the otheralong the linear feed path to force them together; (b) means forreciprocating the wire in the axial direction; (c) means for setting acycle pattern of the feed speed of the workpiece or the wire whichrecurs in a sequence of continuing at a constant speed, accelerating,continuing at the constant speed, decelerating, and continuing at theconstant speed, and a cycle pattern of the reciprocating speed of thewire which recurs in a sequence of stopping, accelerating, continuing ata constant speed, decelerating, and stopping, the two cycle patternsbeing correlated with each other much that at least the continuing atthe constant speed mode on the low speed side of the feed speed cyclepattern and the stopping of the reciprocating speed cycle pattern aresynchronized with each other; and (d) means for controlling theoperation of the feeding means and the reciprocating means according tothe first and second cycle patterns set by the setting means.

In the fourth and fifth preferred forms, the two cycle patterns arecorrelated such that at least the continuing at a constant speed mode onthe low speed side of the feed speed cycle pattern and the stopping ofthe reciprocating speed cycle pattern are synchronized with each other,and a time period during which the feed speed is in the continuing atthe constant speed mode on a high speed side and a time period duringwhich the reciprocating speed is in the continuing at the constant speedmode are synchronized with each other.

In a sixth preferred form of the present invention, there is provided awire saw slicing apparatus of the type in which a plurality of parallel,regularly spaced lines of a wire and a workpiece are relatively fedtoward each other along a linear feed path to force them together whilereciprocating the wire in the axial direction to slice the workpieceinto wafers, with a slurry supplied to a contact area between the wireand the workpiece, the wire saw slicing apparatus comprising: (a) meansfor feeding one of the workpiece and the wire relative to the otheralong the linear feed path to force them together; (b) means forreciprocating the wire in the axial directions; (c) means for setting acycle pattern of the feed speed of the workpiece or the wire whichrecurs in a sequence of accelerating, decelerating, accelerating, and acycle pattern of the reciprocating speed of the wire which recurs in asequence of stopping, accelerating, decelerating, and stopping, the twocycle patterns being correlated with each other such that at least atransitional part of the feed speed cycle pattern between thedecelerating and the accelerating and the stopping of the reciprocatingspeed cycle pattern are synchronized with each other; and (d) means forcontrolling the operation of the feeding means and the reciprocatingmeans according to the first and second cycle patterns set by thesetting means.

In the sixth preferred form, one of the two cycle patterns may furtherhave a mode of continuing at a constant speed provided between theaccelerating and the decelerating.

In the fourth and fifth preferred forms of the present invention, thetwo cycle patterns set by the setting means are preferably composed of arepetition of a unit pattern. It is further preferable that each of thetwo cycle patterns set by the setting means are basically composed of arepetition of a unit pattern, and the feed speed of the workpiece or thewire in a transitional part of the feed speed cycle pattern between theaccelerating and the decelerating varies inversely with a contact lengthbetween the workpiece and the wire.

The feed means preferably includes a drive motor rotatable to feed oneof the workpiece and the wire relatively to each other along the linearfeed path, and the reciprocating means preferably includes a drive motorrotatable to reciprocate the wire in the axial direction of the same.The controlling means preferably comprises a first controller forcontrolling the rotational speed of the drive motor of the feed meansaccording to the feed speed cycle pattern set by the setting means, anda second controller for controlling the rotational speed of the drivemotor of the reciprocating means according to the reciprocating speedcycle pattern net by the setting means.

In another aspect the invention provides a wire saw slicing method ofthe type in which a plurality of parallel, regularly spaced lines of awire and a workpiece are relatively fed toward each other along a linearfeed path to force them together while reciprocating the wire in theaxial direction to slice the workpiece into wafers, with a slurrysupplied to a contact area between the wire and the workpiece, the wiresaw slicing method comprising the steps of; (a) feeding one of theworkpiece and the wire relative to the other along the linear feed pathto force them together at a speed having a first cycle pattern recurringin sequence of stopping, accelerating, continuing at a constant speed,decelerating, and stopping); (b) reciprocating the wire in the axialdirection at a speed having a second cycle pattern recurring in asequence of stopping, accelerating, continuing at a constant speed,decelerating, and stopping; and (c) synchronizing the first cyclepattern and the second cycle pattern at least through a time periodduring which the feed speed and the reciprocating speed continue at therespective constant speeds.

As a second preferred form of the method of this invention, there isprovided a wire saw slicing method of the type in which a plurality ofparallel, regularly spaced lines of a wire and a workpiece arerelatively fed toward each other along a linear feed path to force themtogether while reciprocating the wire in the axial direction to slicethe workpiece into wafers, with a slurry supplied to a contact areabetween the wire and the workpiece, the wire saw slicing methodcomprising the steps of: (a) feeding one of the workpiece and the wirerelative to the other along the linear feed path to force them togetherat a speed having a first cycle pattern recurring in a sequence ofstopping, accelerating, continuing at a constant speed, decelerating,and stopping; (b) reciprocating the wire in the axial direction at aspeed having a second cycle pattern recurring in a sequence of stopping,accelerating, continuing at a constant speed, decelerating, andstopping; and (c) synchronising the first cycle pattern and the secondcycle pattern at least at the time of the stopping of the respectivespeeds.

In the first and second preferred forms of the method of this invention,the first and second cycle patterns are preferably synchronized at leastat the time of the stopping of the respective speeds and through a timeperiod during which the feed speed and the reciprocating speed continueat the respective constant speeds.

As a third preferred form of the method, the invention provides a wiresaw slicing method of the type in which a plurality of parallel,regularly spaced lines of a wire and a workpiece are relatively fedtoward each other along a linear feed path to force them together whilereciprocating the wire in the axial direction to slice the workpieceinto wafers, with a slurry supplied to a contact area between the wireand the workpiece, the wire saw slicing method comprising the steps of:(a) feeding one of the workpiece and the wire relative to the otheralong the linear feed path to force them together at a speed having afirst cycle pattern recurring in a sequence of stopping, accelerating,decelerating, and stopping; (b) reciprocating the wire in the axialdirection at a speed having a second cycle pattern recurring in asequence of stopping, accelerating, decelerating, and stopping; and (c)synchronizing the first cycle pattern and the second cycle pattern atleast at the time of the stopping of the respective speeds.

In the third preferred form of the method of this invention, one of thefirst and second cycle patterns may further have a mode of continuing ata constant speed provided between the accelerating and the decelerating.

A fourth preferred form of the method of this invention is characterizedby a wire saw slicing method of the type in which a plurality ofparallel, regularly spaced lines of a wire and a workpiece arerelatively fed toward each other along a linear feed path to force themtogether while reciprocating the wire in the axial direction to slicethe workpiece into wafers, with a slurry supplied to a contact areabetween the wire and the workpiece, the wire saw slicing methodcomprising the steps of: (a) feeding one of the workpiece and the wirerelative to the other along the linear feed path to force them togetherat a speed having a first cycle pattern recurring in a sequence ofcontinuing at a constant speed, accelerating, continuing at the constantspeed, decelerating, and continuing at the constant speed; (b)reciprocating the wire in the axial direction at a speed having a secondcycle pattern recurring in a sequence of stopping, accelerating,continuing at a constant speed, decelerating, and stopping; and (a)synchronizing the first and second cycle patterns such that at least atime period during which the feed speed is in the continuing at theconstant speed mode on the high speed side and a time period duringwhich the reciprocating speed is in the continuing at the constant speedmode are synchronized with each other.

A fifth preferred form of the method of this invention in characterizedby a saw slicing method of the type in which a plurality of parallel,regularly spaced lines of a wire and a workpiece are relatively fedtoward each other along a linear feed path to force them together whilereciprocating the wire in the axial direction to slice the workpieceinto wafers, with a slurry supplied to a contact area between the wireand the workpiece, the wire saw slicing method comprising the steps of:(a) feeding one of the workpiece and the wire relative to the otheralong the linear feed path to force them together at a speed having afirst cycle pattern recurring in a sequence of continuing at a constantspeed, accelerating, continuing at the constant speed, decelerating, andcontinuing at the constant speed; (b) reciprocating the wire in theaxial direction at a speed having a second cycle pattern recurring in asequence of stopping, accelerating, continuing at a constant speed,decelerating, and stopping, and (c) synchronizing the first and secondcycle patterns such that at least the continuing at a constant speedmode on the low speed side of the feed speed and the stopping of thereciprocating speed are synchronized with each other.

In the fifth preferred form of the method, the first and second cyclepatterns are preferably synchronized such that at least a time periodduring which the feed speed is in the continuing at the constant speedmode on the high speed side and a time period during which thereciprocating speed is in the continuing at the constant speed mode aresynchronized with each other, and the continuing at a constant speedmode on the low speed side of the first cycle pattern and the stoppingof the second cycle pattern are synchronized with each other.

A sixth preferred form of the method of this invention is characterizedby a wire saw slicing method of the type in which a plurality ofparallel, regularly spaced lines of a wire and a workpiece arerelatively fed toward each other along a linear feed path to force themtogether while reciprocating the wire in the axial direction to slicethe workpiece into wafers, with a slurry supplied to a contact areabetween the wire and the workpiece, the wire saw slicing methodcomprising the steps of: (a) feeding one of the workpiece and the wirerelative to the other along the linear feed path to force them togetherat a speed having a first cycle pattern recurring in a sequence ofaccelerating, decelerating, and accelerating; (b) reciprocating the wirein the axial direction at a speed having a second cycle patternrecurring in a sequence of stopping, accelerating, decelerating, andstopping; and (c) synchronizing the first and second cycle patterns suchthat at least a transitional part of the first cycle pattern between thedecelerating and the accelerating and the stopping of the second cyclepattern are synchronized with each other.

In the sixth preferred form of the method, one of the first and secondcycle patterns may further have a mode of continuing at a constant speedprovided between the accelerating and the decelerating.

The workpiece to be sliced by the apparatus and method of the presentinvention is composed, for example, of a semiconductor single crystalingot.

In view of the abrasive wear and cutting efficiency of a wire, theconventionally used wire saw slicing apparatus cuts off or slices aworkpiece by reciprocating the wire in a like manner as a saw used forcutting lumbers.

To reciprocate the wire, a cycle pattern recurring in a sequence ofstopping, accelerating, continuing at a constant speed, decelerating,and stopping is used instead of a mere repetition of alternatingstopping and constant speed modes. It is also possible to employ anothercycle pattern which recurs in a sequence of stopping, accelerating,decelerating, and stopping.

The slicing speed which is represented by a speed of downward movementof the workpiece or a speed of linear movement of the wire is constantin general. In a particular case, however, the slicing speed may bevaried linearly according to a certain principle.

Accordingly, even when the reciprocating movement of the wire is stoppedor continues at a slow speed, the workpiece or the wire is fed linearlyat the same speed an provided when the wire is reciprocating at aconstant speed. Consequently, sliced wafer necessarily have adeformation of undulations (waviness) corresponding to the period ofreciprocation of the wire. The deformation of undulations (waviness)cannot be removed by a subsequent working process such as lapping butlowers the yield of semiconductor devices formed on the sliced wafer.

The foregoing problems are solved by the present invention in which thelinear feed speed of the workpiece or the wire is changed in synchronismwith the period of reciprocation of the wire to produce as-sliced wafershaving no undulations (waviness) on the sliced surface.

The linear feed speed of the workpiece or the wire has a cycle pattern,and the reciprocating speed of the wire has a cycle pattern. The twocycle patterns are synchronized with each other at least at the time ofstopping of the respective speeds and through a time period during whichthe respective feed speeds continue at their constant speeds. Moreconcretely, through a time period during which the reciprocatingmovement of the wire is accelerated from stopping to a constant speed,or decelerated from the constant speed, the linear feed of the workpieceor the wire is temporarily stopped or slowed down to an extent notaffecting the slicing operation.

A further advantageous effect attainable by the foregoing arrangement ofthe present invention is that a slack control as required for the wirein the conventional slicing process is of no importance to the wire ofthe present invention because a sag or slack in the wire can be taken upas the wire restores its original state under the resiliency thereofwhile the linear movement of the workpiece or the wire is stopped orcontinues at a low speed. Thus, the wire of the present invention issubstantially free from breakage.

The above and other objects, features and advantage of the presentinvention will become manifest to those versed in the art upon makingreference to the detailed description and the accompanying sheets ofdrawings in which preferred structural embodiments of the invention areshown in by way of illustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partly pictorial block diagram showing the generalconstruction of a wire saw slicing apparatus according to a firstembodiment of the present invention;

FIG. 2 is a diagrammatical perspective view of a main portion of thewire saw slicing apparatus;

FIG. 3 is a graph showing one example of the cycle pattern of theworkpiece slicing speed;

FIG. 4 is a graph showing one example of the wire reciprocating speed;

FIG. 5 is a graphical representation of the condition in which the cyclepattern of the workpiece slicing speed and the cycle pattern of the wirereciprocating speed are synchronized with each other;

FIG. 6 is a graph showing a surface configuration of an as-sliced wafer;

FIG. 7 is a graph showing another example of the cycle pattern of theworkpiece slicing speed;

FIG. 8 is a graph showing still another example of the workpiece slicingspeed; and

FIG. 9 is a graph showing a surface configuration of a wafer slicedaccording to a conventional method.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described below in greater detail withreference to a first embodiment shown in FIGS. 1 through 5.

As shown in FIG. 2, a wire saw slicing apparatus according to the firstembodiment of this invention includes three plastics main rollers 10A,10B and 10C of the identical construction disposed with their axesparallel spaced from one another, and a wire 12 wound spirally aroundhelical groove 14a, 14b and 14c formed at regular intervals or pitchesin the respective outer peripheral surfaces of the main rollers 10A-10C.The main rollers may be plural in number and should by no means belimited to any particular number, but four or three main rollers as inthe illustrated embodiment are used in general. The main roller 10Cconstitutes a drive roller and is connected in driven relation to adrive motor 16. A rotary motion of the main roller 10C in transmittedvia the wire 12 to the remaining main rollers 10A, 10B which constitutedriven rollers.

The wire 12 has one or a leading end portion wound around a wire take-updrum 22 via a tension adjustment mechanism 20. The wire take-up drum 22is rotatably driven by a torque motor 24. A tension on a portion of thewire 12 extending between the tension adjustment mechanism 20 and thewire take-up drum 22 is regulated according to a voltage applied to thetorque motor 24. A tension on a portion of the wire 12 running betweenthe tension adjustment mechanism 20 and the drive roller 10C is adjustedat a constant value by the tension adjustment mechanism 20.

Similarly, the opposite or a trailing end portion of the wire 12 inwound around a wire take-up drum 32 via a tension adjustment mechanism30, The wire take-up drum 32 in rotatably driven by a torque motor 34. Atension on a portion of the wire 12 extending between the tensionadjustment mechanism 30 and the wire take-up drum 32 is regulatedaccording to a voltage applied to the torque motor 34. A tension on aportion of the wire 12 running between the tension adjustment mechanism30 and the drive roller 10C is adjusted at a constant value by thetension adjustment mechanism 30.

A workpiece 40 is composed, for example, of a semi-conductor singlecrystal ingot having a flat orientation 15 and attached by bonding to aworkpiece holder 42 via the orientation flat. The workpiece holder 42 isvertically moved up and down along a linear path by means of anarrangement shown in FIG. 1.

As shown in FIG. 1, a vertical shaft 44A is connected at one end to anupper surface of the workpiece holder 42 in proximity of one end of theworkpiece holder 42. The vertical shaft 44A slidably extends through afixed vertical guide cylinder 46A for vertical reciprocation relative tothe fixed guide cylinder 46A. A similar vertical shaft 44B is connectedat one end to the upper surface of the workpiece holder 42 in proximityof the other end of the workpiece holder 42. The vertical shaft 44Bslidably extends through a fixed vertical guide cylinder 46B forvertical reciprocation relative to the fixed guide cylinder 46B. Avertical feed screw 48 is connected at one end to a central portion ofthe upper surface of the workpiece holder 42. The feed screw 48 inthreaded with an internally threaded hole (not shown) in a gear 50. Thegear 50 is rotatably driven by a workpiece feed motor 54 via a speedreducer 52.

A workpiece holder feed motor controller 56 controls a rotational speedof the workpiece feed motor 54 according to rotational speed patternsset for the workpiece feed motor 54 in a program-setting unit or setter58. Similarly, a drive motor controller 60 controls a rotational speedof the drive motor 16 according to rotational speed patterns set for thedrive motor 16 in the program setter 58.

The wire saw slicing apparatus of the foregoing construction operates asfollows.

The workpiece feed motor 54 is driven to lower the workpiece holder 42toward a plurality of parallel regularly spaced lines or portions of thewire 12 until the workpiece 40 held on the workpiece holder 42 is incontact with the wire 12 (FIG. 2). At the same time, the drive roller10C is rotated by the drive motor 16 to reciprocate the wire 12 in theaxial or longitudinal direction thereof. A slurry (a working fluidcontaining abrasive grain) is supplied to a contact area between theworkpiece 40 and the wire 12. While keeping this condition, theworkpiece 40 is further moved downwards whereby the workpiece 40 insliced at one time into a multiplicity of wafers by a lapping actionattained by the reciprocating wire 12 and the slurry supplied thereto.

FIG. 3 shows one example of a cycle pattern of the working or slicingspeed of the workpiece 40 which is represented by the relation betweenthe slicing speed and the number of cycles taken after the start of aslicing operation. The program setter 58 contains a workpiece feed motorrotational speed pattern (a pattern of rotational speed of the workpiecefeed motor 54) which corresponds to the cycle pattern shown in FIG. 3.In the illustrated example, the slicing speed has a cycle time of onehour, and within the one-hour cycle time, the slicing speed goes up from0 μm/min to 600 μm/min, subsequently continues at 600 μm/min for apredetermined period of time A, and thereafter goes down from 600 μm/minto 0 μm/min. Such a unit cycle pattern is repeated 6 times to slice oneworkpiece 40. In a first embodiment of the present invention, the cyclepattern of workpiece slicing speed needs to have a pattern recurring ina sequence of stopping, accelerating, continuing at a constant speed,decelerating, and stopping. The cycle time or period of the workpieceslicing speed cycle pattern has no particular limitation, so that anydesired cycle pattern can be set for the workpiece slicing speed.

FIG. 4 illustrates one example of a cycle pattern of the wirereciprocating speed. The program setter 58 contains a drive motorrotational speed pattern (a pattern of rotational speed of the drivemotor 16) which corresponds to the illustrated wire reciprocating speedpattern. In the illustrated example, the wire reciprocating speed has acycle time of one hour. In a first one-hour cycle time or period, thewire reciprocating speed goes up from 0 m/min to 400 m/min for a forwardstroke or movement in one or the left-hand direction, subsequentlycontinues at 400 m/min for a predetermined period of time B, andthereafter goes down from 400 m/min to 0 m/min for a backward stroke ormovement in the opposite or the right-hand direction. In the next followone-hour cycle time, the wire reciprocating speed goes up from 0 m/minto 400 m/min for a forward stroke or movement in one or the right-handdirection, subsequently continues at 400 m/min for a predeterminedperiod of time B, and thereafter goes down from 400 m/min to 0 m/min fora backward stroke or movement in the opposite or the left-handdirection.

The cycle pattern in the first cycle time and the cycle pattern in thenext following cycle time are identical with each other but opposite indirection. In the example shown in FIG. 4, the cycle pattern in eachdirection of the wire reciprocating speed is repeated three times toslice one workpiece 40. In the first embodiment of the presentinvention, the cycle pattern of wire reciprocating speed needs to have apattern recurring in a sequence of stopping, accelerating, continuing ata constant speed, decelerating, and stopping. The cycle time or periodof the wire reciprocating speed cycle pattern has no particularlimitation, so that any desired cycle pattern can be set for the wirereciprocating speed.

One important feature of the present invention resides in that the cyclepattern of the workpiece slicing speed, namely the cycle pattern of thefeed speed of the workpiece relative to the wire along a linear path,and the cycle pattern of the wire reciprocating speed are synchronizedwith each other.

The aforesaid synchronization between the cycle pattern of the workpieceslicing speed and the cycle pattern of the wire reciprocating speedmeans that, as shown in FIG. 5, at least the time period A during whichthe workpiece slicing speed continues at a constant value (600 μm/min inthe example shown in FIG. 5) and the time period B during which the wirereciprocating speed continues at a constant value (400 m/min in theexample shown in FIG. 5) are synchronized with each other.

The workpiece slicing speed and the wire reciprocating speed also haveadditional items that can be synchronized with each other, namely, thestopping time period, the accelerating time period, and the deceleratingtime period. It is most preferable that the workpiece is stopped whilethe wire is stopping. However, the workpiece may be moved or fed at alow speed while the wire is stopping. It is also preferable that theworkpiece is stopped while the wire is accelerating or decelerating. Asan alternative, the workpiece may be moved or fed at a low speed whilethe wire it in accelerating or decelerating.

It was experimentally proved that as-sliced wafers having an ideal shape(i.e., a flat wafer) with negligible irregularities or undulations(waviness) on the sliced surface, such as shown in FIG. 6, could beobtained by a slicing operation performed under the condition in whichat least the time period A and the time period B are synchronized witheach other. On the contrary, it was also proved by experiment that aslicing operation achieved with the workpiece slicing speed cyclepattern shown in FIG. 3 combined asynchronously with the wirereciprocating speed pattern of FIG. 4 produced wafers having noticeableundulations (waviness) on the sliced surface, as shown in FIG. 9. FIGS.6 and 9 each illustrate the result of a measurement on the surfaceroughness of a sliced wafer using a surface roughness tester. In each ofFIGS. 6 and 9, the vertical axis represents undulations (waviness) onthe measured wafer surface, and the horizontal axis represents measuredpositions on the wafer surface taken in the slicing direction.

FIG. 7 shows another example of the workpiece slicing speed cyclepattern taken over several numbers of cycles after the start of aslicing operation. In this example, the maximum workpiece slicing speedin each cycle time or period decreases with an increase in effectivediameter "d" of the workpiece 40 being sliced (or a contact lengthbetween the wire 12 and the workpiece 40 being sliced) so that slicingresistance in the individual cycles can be made substantially equalthroughout the slicing operation.

The workpiece slicing speed cycle pattern thus arranged is able toequalize the amplitude of undulations (waviness) formed during eachcycle time on the surface of a sliced wafer 40, thereby reducing orsmoothing the undulations (waviness) to a greater extent than anobtained the cycle pattern shown in FIG. 3.

The workpiece slicing speed cycle pattern shown in FIG. 7, when combinedwith the wire reciprocating speed cycle pattern of FIG. 4 in a likesynchronized condition or state as shown in FIG. 5, the sameadvantageous effects of the present invention can be also attained.

FIG. 8 illustrates still another example of the workpiece slicing speedcycle pattern taken over several cycles after the start of a slicingoperation. In thin example, the workpiece slicing speed i set toperiodically change in the range of 200 gm/min to 540 m/min, with anaverage slicing speed kept higher than that of the cycle pattern shownin FIG. 3.

The workpiece slicing speed cycle pattern shown in FIG. 8 may becombined in a synchronized state (not shown) with the wire reciprocatingspeed cycle pattern of FIG. 4, in which instance the same advantageouseffects of the invention as described above can be achieved.

It will be appreciable that many variations of the workpiece slicingspeed cycle patterns and the wire reciprocating speed cycle patterndescribed and illustrated herein are possible in the light of the aboveteaching. All such variations an reasonably and properly come within thescope of the technical idea of the present invention may be combined invarious appropriate manners to attain the same advantageous effects asdescribed above.

In the illustrated embodiments, the wire saw slicing apparatus is soconstructed as to feed the workpiece along a linear path to force thesame against the wire. The present invention is also useful whenembodied in a wire saw slicing apparatus of the type in which the wireis fed along a linear path and forced against the workpiece during theslicing process.

As described above, according to the present invention, a workpiece suchas a silicon single crystal ingot can be sliced into a multiplicity ofwafers with no surface undulations (waviness) and kerf lose involved andwith good yield resulting therefrom.

Obviously, various minor changes and modifications of the presentinvention are possible in the light of the above teaching. It istherefore to be understood that within the scope of appended claims theinvention may be practiced otherwise than as specifically described.

What is claimed is:
 1. A wire saw slicing method in which a plurality ofparallel, regularly spaced lines of a wire and a workpiece arerelatively fed toward each other along a linear feed path to force thewire and workpiece together while reciprocating the wire in an axialdirection to slice the workpiece into wafers, with a slurry supplied toa contact area between the wire and the workpiece, said wire saw slicingmethod comprising the steps of:(a) feeding one of the workpiece and thewire relative to each other along the linear feed path to force theworkpiece and the wire together at a speed having a first cyclicpattern, said first cyclic pattern having a plurality of modes whichform one cycle of the first cyclic pattern; (b) reciprocating the wirein the axial direction at a reciprocating speed having a second cyclicpattern, said second cyclic pattern having a plurality of modes whichform one cycle of the second cyclic pattern; and (c) synchronizing saidfirst cyclic pattern and said second cyclic pattern at least during atime period of one of the modes of the respective cyclic patterns.
 2. Awire saw slicing method according to claim 1, wherein said first cyclicpattern recurs in a mode sequence of stopping, accelerating, continuingat a constant speed, decelerating and stopping, said second cyclicpattern recurs in a mode sequence of stopping, accelerating, continuingat a constant speed, decelerating, and stopping, said synchronizing stepis effected at least through a time period during which said feed speedand said reciprocating speed continue at the respective constant speeds.3. A wire saw slicing method according to claim 1, wherein said firstcyclic pattern recurs in a mode sequence of stopping, accelerating,continuing at a constant speed, decelerating and stopping, said secondcyclic pattern recurs in a mode sequence of stopping, accelerating,continuing at a constant speed, decelerating, and stopping, and saidsynchronizing step is effected at least at the time of said stopping ofthe respective speeds.
 4. A wire saw slicing method according to claim1, wherein said first cyclic pattern recurs in a mode sequence ofstopping, accelerating, continuing at a constant speed, decelerating,and stopping, said second cyclic pattern recurs in a mode sequence ofstopping, accelerating, continuing at a constant speed, decelerating,and stopping, and said synchronizing step is effected at least at thetime of said stopping of the respective speeds and through a time periodduring which said feed speed and said reciprocating speed continue atthe respective constant speeds.
 5. A wire saw slicing method accordingto claim 1, wherein said first cyclic pattern recurs in a mode sequenceof stopping, accelerating, decelerating, and stopping, said secondcyclic pattern recurs in a mode sequence of stopping, accelerating,decelerating, and stopping, and said synchronizing step is effected atleast at the time of said stopping of the respective speeds.
 6. A wiresaw slicing method according to claim 5, wherein one of said first andsecond cyclic patterns further has a mode of continuing at a constantspeed provided between said accelerating and said decelerating.
 7. Awire saw slicing method according to claim 1, wherein said first cyclicpattern recurs in a mode sequence of continuing at a constant speed,accelerating, continuing at the constant speed, decelerating andcontinuing at the constant speed, said second cyclic pattern recurs in amode sequence of stopping, accelerating, continuing at a constant speed,decelerating, and stopping, and said synchronizing step is effected suchthat at least a time period during which said feed speed is in saidcontinuing at the constant speed mode on a high speed side and a timeperiod during which said reciprocating speed is in said continuingconstant speed mode are synchronized with each other.
 8. A wire sawslicing method according to claim 1, wherein said first cyclic patternrecurs in a mode sequence of continuing at the constant speed,accelerating, continuing at a constant speed, decelerating, andcontinuing at the constant speed, said second cyclic pattern recurs in amode sequence of stopping, accelerating, continuing at a constant speed,decelerating, and stopping, and said synchronizing step is effected suchthat at least said continuing at a constant speed mode on a low speedside of the feed speed and said stopping of the reciprocating speed aresynchronized with each other.
 9. A wire saw slicing method according toclaim 1, wherein said first cyclic pattern recurs in a mode sequence ofcontinuing at a constant speed, accelerating, continuing at the constantspeed, decelerating, and continuing at the constant speed, said secondcyclic pattern recurs in a mode sequence of stopping, accelerating,continuing at a constant speed, decelerating, and stopping and saidsynchronizing step is effected such that at least a time period duringwhich said feed speed is in said continuing at the constant speed modeon a high speed side and a time period during which said reciprocatingspeed is in said continuing at the constant speed mode are synchronizedwith each other, and said continuing at the constant speed mode on a lowspeed side of said first cyclic pattern and said stopping of said secondcyclic pattern are synchronized with each other.
 10. A wire saw slicingmethod according to claim 1, wherein said first cyclic pattern recurs ina mode sequence of accelerating, decelerating, and accelerating, saidsecond cyclic pattern recurs in a mode sequence of stopping,accelerating, decelerating, and stopping and said synchronizing step iseffected such that at least a transitional part of said first cyclicpattern between said decelerating and said accelerating and saidstopping of said second cyclic pattern are synchronized with each other.11. A wire saw slicing method according to claim 10, wherein one of saidfirst and second cyclic patterns further has a mode of continuing at aconstant speed provided between said accelerating and said decelerating.